Process for the production of aliphatic-aromatic polyesters

ABSTRACT

This invention relates to a process for the production of polyesters comprising an esterification/transesterification stage and a polycondensation stage, characterised in that the said polycondensation stage is carried out in the presence of a catalyst comprising a mixture of at least one Titanium-based compound and at least one Zirconium-based compound in which the Ti/(Ti+Zr) ratio by weights is equal to or greater than 0.01 and equal to or less than 0.70.

This invention relates to a process for the production of polyesterscomprising an esterification or transesterification stage and apolycondensation stage, characterised in that the said polycondensationstage is carried out in the presence of a catalyst comprising a mixtureof at least one compound based on Titanium and at least one compoundbased on Zirconium in which the Ti/(Ti+Zr) ratio by weight is equal toor greater than 0.01 and equal to or less than 0.70, preferably equal toor greater than 0.02 and equal to or less than 0.60.

This process makes it possible to obtain aliphatic-aromatic polyestershaving suitable thermal stability and terminal acidity properties with areduced content of residual cyclic oligomers, which are undesiredby-products of the polymerisation process. Because of this thesepolyesters are particularly suitable for use in the production of films,expanded articles and moulded articles and in general in all sectors ofthe plastics industry.

Processes for the production of polyesters have hitherto been widelyknown in the literature and exploited industrially. On an industrialscale these processes generally comprise at least two reaction stages: afirst esterification or transesterification stage in which, startingfrom diols of dicarboxylic acids, their salts or their esters, anintermediate oligomer product is formed, and this is subsequently causedto react in a polycondensation stage in such a way as to obtain thefinal polyester.

In comparison with the production of wholly aromatic polyesters, such asPET and PBT, the production of aliphatic-aromatic polyesters presentsspecial problems associated with the different nature of the monomersused and the reaction conditions required in order to obtain polymershaving sufficiently high molecular weights.

In particular processes for the production of aliphatic-aromaticpolyesters have disadvantages associated with the formation ofsignificant quantities of residual cyclic oligomers, which by adverselyinfluencing the properties of the polyesters and the productivity of theprocesses make it necessary to use suitable equipment for theirseparation and recovery of the polyester. In addition to this, in orderto achieve high molecular weights these processes require relativelylong reaction times and high reaction temperatures, which give rise topolyester degradation phenomena. Under these conditions the terminalacidity of the polyesters tends to increase, compromising their thermalstability properties in subsequent processing stages. Materials havingpoor thermal stability properties can in fact give rise to materialswhich tend to lose mechanical properties over time, when for exampleused for the production of blown films. As a consequence of this it iscustomary to subject polyesters of this type to chain extension orhydrolysis stabilisation treatments which attempt to limit thesedegradation phenomena, typically through attaching suitable derivativesat the terminal acids of the polyesters. This however gives rise to theprovision of additional stages of polyester post-treatment, and thus hasan adverse effect on the productivity of the processes.

There is therefore a need to identify production processes foraliphatic-aromatic polyesters which overcome the abovementioned problemsof residual cyclic oligomer formation and which help to limit thethermal instability and excessive terminal acidity problems.

Starting from this requirement it has surprisingly been discovered thatthese problems can be significantly limited through suitable selectionof the catalyst used during the polycondensation stage. It has in factbeen discovered that a catalyst comprising a mixture of at least oneTitanium-based compound and at least one Zirconium-based compound inwhich the Ti/(Ti+Zr) ratio by weight is equal to or greater than 0.01and equal to or less than 0.70, preferably equal to or greater than 0.02and equal to or less than 0.60, makes it possible to obtainaliphatic-aromatic polyesters having sufficiently high molecularweights, suitable thermal stability properties and terminal acidity,while at the same time giving rise to the formation of significantlysmaller quantities of residual cyclic oligomers.

In particular this invention relates to a process for the production ofpolyesters comprising:

a) a dicarboxylic component comprising:

-   -   a1) 40-80% mol, with respect to the total dicarboxylic        component, of units deriving from at least one aromatic        dicarboxylic acid,    -   a2) 20-60% mol, with respect to the total dicarboxylic        component, of units deriving from at least one aliphatic        dicarboxylic acid,

b) a diol component comprising units deriving from at least onealiphatic diol;

the said process comprising an esterification or transesterificationstage and a subsequent polycondensation stage and being characterised inthat the said polycondensation stage is performed in the presence of acatalyst comprising a mixture of at least one Titanium-based compoundand at least one Zirconium-based compound in which the Ti/(Ti+Zr) ratioby weight is equal to or greater than 0.01 and equal to or less than0.70 preferably equal to or greater than 0.02 and equal to or less than0.60.

With regard to the aliphatic-aromatic polyesters produced by the processaccording to this invention, these comprise a diol component derivingfrom at least one aliphatic diol selected from 1,2-ethanediol,1,2-propanediol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol,1,6-hexanediol, 1,7-hheptanediol, 1,8-octanediol, 1,9-noenandiol,1,10-decanediol, 1,11-undecanediol, 1,12-dodecanediol,1,13-tridecanediol, 1,4-ciclohexanedimethanol, neopentyl glycol,2-methyl-1,3-propanediol, dianhydrosorbitol, dianhydromannitol,dianhydroiditol, cyclohexanediol, cyclohexanemethanediol,pentaerythritol, glycerol, polyglycerol, trimethylolpropane, dialkyleneglycols and polyalkylene glycols having a molecular weight of 100-4000,such as for example polyethylene glycol, polypropylene glycol and theirmixtures. Preferably the diol component comprises at least 50% moles ofone or more diols selected from 1,2-ethanediol, 1,3-propanediol,1,4-butanediol. More preferably, the diol component comprises orconsists of 1,2-ethanediol, 1,4-butanediol or mixtures thereof.

The dicarboxylic component of the polyesters produced by the processaccording to this invention comprises 40-80% mol, preferably more than40% mol, more preferably 45-60% mol, with respect to the totaldicarboxylic component, of units deriving from at least one aromaticdicarboxylic acid and 20-60% mol, preferably less than 60% mol, morepreferably 40-55% mol, with respect to the total dicarboxylic component,of units deriving from at least one aliphatic dicarboxylic acid.

The aromatic dicarboxylic acids are advantageously selected fromterephthalic acid, isophthalic acid, 2,5-furandicarboxylic acid, theiresters, salts and mixtures. In a preferred embodiment, the said aromaticdicarboxylic acids comprise:

-   -   -   from 1 to 99% mol, preferably from 5 to 95% and more            preferably from 10 to 80% of terephthalic acid, its esters            or its salts;        -   from 99 to 1% mol, preferably from 95 to 5% and more            preferably from 90 to 20% of 2,5-furandicarboxylic acid, its            esters or its salts.

The aliphatic dicarboxylic acids are advantageously selected fromsaturated C₂-C₂₄ dicarboxylic acids, preferably C₄-C₁₃, more preferablyC₄-C₁₁, their C₁-C₂₄, preferably C1-C4, alkyl esters, their salts andtheir mixtures. Preferably the aliphatic dicarboxylic acids are selectedfrom: succinic acid, 2-ethylsuccinic acid, glutaric acid,2-methylglutaric acid, adipic acid, pimelic acid, suberic acid, azelaicacid, sebacic acid, undecanedioic acid, dodecanedioic acid, brassylicacid and their C₁₋₂₄ alkyl esters. In a preferred embodiment of thisinvention the aliphatic dicarboxylic acids comprise mixtures comprisingat least 50% mol, preferably more than 60% mol, more preferably morethan 65% mol, of succinic acid, adipic acid, azelaic acid, sebacic acid,brassylic acid, their C₁-C₂₄, preferably C₁-C₄ esters and theirmixtures.

In a preferred embodiment of the process according to this invention thepolyester produced is advantageously selected from:

(A) polyesters comprising repetitive units deriving from aromaticdicarboxylic acids of the phthalic acid type, preferably terephthalicacid, aromatic dicarboxylic acids and aliphatic diols (AAPE-A),characterised by an aromatic units content of between 40-65.% mol,preferably between 45-60% mol with respect to the total moles of thedicarboxylic component.

AAPE-A polyesters are preferably selected from: poly(1,4-butyleneadipate-co-1,4-butylene terephthalate), poly(1,4-butylenesebacate-co-1,4-butylene terephthalate), poly(1,4-butyleneazelate-co-1,4-butylene terephthalate), poly(1,4-butylenebrassylate-co-1,4-butylene terephthalate), poly(1,4-butylenesuccinate-co-1,4-butylene terephthalate), poly(1,4-butyleneadipate-co-1,4-butylene sebacate-co-1,4-butylene terephthalate),poly(1,4-butylene azelate-co-1,4-butylene sebacate-co-1,4-butyleneterephthalate), poly(1,4-butylene adipate-co-1,4-butyleneazelate-co-1,4-butylene terephthalate), poly(1,4-butylenesuccinate-co-1,4-butylene sebacate-co-1,4-butylene terephthalate),poly(1,4-butylene adipate-co-1,4-butylene succinate-co-1,4-butyleneterephthalate), poly(1,4-butylene azelate-co-1,4-butylenesuccinate-co-1,4-butylene terephthalate).

(B) polyesters comprising repetitive units deriving from aromaticheterocyclic dicarboxylic compounds, preferably 2,5-furandicarboxylicacid, aromatic dicarboxylic acids and aliphatic diols (AAPE-B),characterised by an aromatic units content of between 40-80% mol,preferably between 45-75% mol with respect to the total moles of thedicarboxylic component.

AAPE-B polyesters are preferably selected from: poly(1,4-butyleneadipate -co-1,4-butylene-2,5 -furandicarboxylate), poly(1,4-butylenesebacate-co-1,4-butylene-2,5-furandicarboxylate), poly(1,4-butyleneazelate-co-1,4-butylene-2,5-furandicarboxylate), poly(1,4-butylenebrassylate-co-1,4-butylene-2,5 -furandicarboxylate), poly(1,4-butylenesuccinate-co-1,4-butylene-2,5-furandicarboxylate), poly(1,4-butyleneadipate-co-1,4-butylenesebacate-co-1,4-butylene-2,5-furandicarboxylate), poly(1,4-butyleneazelate -co-1,4-butylenesebacate-co-1,4-butylene-2,5-furandicarboxylate), poly(1,4-butyleneadipate-co-1,4-butylene azelate-co-1,4-butylene-2,5-furandicarboxylate),poly(1,4-butylene succinate-co-1,4-butylenesebacate-co-1,4-butylene-2,5-furandicarboxylate), poly(1,4-butyleneadipate-co-1,4-butylenesuccinate-co-1,4-butylene-2,5-furandicarboxylate), poly(1,4-butyleneazelate-co-1,4-butylenesuccinate-co-1,4-butylene-2,5-furan-dicarboxylate).

In addition to the dicarboxylic component and the diol component, thepolyesters produced by the process according to this inventionpreferably comprise repetitive units deriving from at least onehydroxyacid in quantities of between 0-49% preferably between 0-30% molwith respect to the total moles of the dicarboxylic component. Examplesof suitable hydroxyacids are glycolic acid, hydroxybutyric acid,hydroxycaproic acid, hydroxyvaleric acid, 7-hydroxyheptanoic acid,8-hydroxycaproic acid, 9-hydroxynonanoic acid, lactic acid or lactides.The hydroxyacids may be inserted into the chain as such or may also bepreventively caused to react with diacids or diols. In the processaccording to this invention, the hydroxyacids are advantageously addedduring the esterification stage.

Long molecules having two functional groups, including those withfunctional groups which are not in the terminal position, may also bepresent in quantities not exceeding 10% mol with respect to the totalmoles of the dicarboxylic component. Examples are dimer acids,ricinoleic acids and acids incorporating epoxy and even polyoxyethylenefunctional groups having a molecular weight of between 200 and 10000. Inthe process according to this invention these long molecules with twofunctional groups are advantageously added during the esterificationstage.

Diamines, amino acids and aminoalcohols may also be present inpercentages up to 30% mol with respect to the total moles of thedicarboxylic component. In the process according to this invention thesediamines, amino acids and aminoalcohols are advantageously added duringthe esterification stage.

During the esterification stage of the process for the preparation ofpolyesters according to this invention, one or more multiple functionalgroup molecules may be added in quantities of between 0.1 and 3% molwith respect to the total moles of the dicarboxylic component (and anyhydroxyacids) in order to obtain branched products. Examples of thesemolecules are glycerol, pentaerythritol, trimethylolpropane, citricacid, dip entaerythritol, monoanhydrosorbitol, monoanhydromannitol, acidtriglycerides or polyglycerols.

The molecular weight Mn of the polyesters obtained by the processaccording to this invention is preferably greater than 20000, morepreferably >30000, even more preferably >50000. As far as thepolydispersity index for the molecular weights Mw/Mn is concerned, thisis instead preferably between 1.5 and 10, more preferably between 1.6and 5 and even more preferably between 1.8 and 2.5

The molecular weights M_(n) and M_(w) may be measured by Gel PermeationChromatography (GPC). The determination may be performed with thechromatography system held at 40° C., using a set of three columns inseries (particle diameter 5 μ and porosities of 500 A, 1000 A and 10000A respectively), a refractive index detector and chloroform as eluent(flow 1 ml/min), using polystyrene as the reference standard.

The polyesters obtained by the process according to this invention havea smaller quantity of residual cyclic oligomers and lower terminalacidity values than similar polyesters obtained using the processes forproduction in the prior art, thanks to use of the catalyst comprising amixture of at least one Titanium-based compound and at least oneZirconium-based compound, in which the Ti/(Ti+Zr) ratio by weight isequal to or greater than 0.01 and equal to or less than 0.70, preferablyequal to or greater than 0.02 and equal to or less than 0.60 in thepolycondensation stage.

With regard to measurement of the cyclic oligomers content, this isperformed gravimetrically after isolation from the polycondensationdistillates. The cyclic oligomers are isolated by extracting a uniformaliquot of the exactly weighed polycondensation distillates with 5volumes of water and 5 volumes of diethyl ether. The organic phase isseparated off, dewatered with sodium sulphate and then dried byevaporation at low pressure, using for example a rotating evaporator toseparate out the pure oligomer. If emulsions form at the interface orthe separation is not sharp, the ionic strength of the aqueous phase canbe increased by adding salts such as KCl, KI or NaCl to encouragebreaking of the emulsion. The oligomer separated out is weighed and thequantity of oligomers is expressed as a ratio by weight with respect tothe quantity of polyester which could theoretically be obtained byconverting all the dicarboxylic acid fed to the process.

The terminal acid groups content may be measured in the following way:1.5-3 g of polyester are placed in a 100 ml flask together with 60 ml ofchloroform. After the polyester has completely dissolved 25 ml of2-propanol are added and, immediately before the analysis, 1 ml ofdeionised water. The solution so obtained is titrated against apreviously standardised solution of KOH in ethanol. An appropriateindicator, such as for example a glass electrode for acid-basetitrations in non-aqueous solvents, is used to determine the end pointof the titration. The terminal acid groups content is calculated on thebasis of the consumption of the KOH solution in ethanol according to thefollowing equation:

${{Terminal}\mspace{14mu} {acid}\mspace{14mu} {groups}\mspace{14mu} {content}\mspace{14mu} \left( {{meq}\mspace{14mu} {KOH}\text{/}{kg}\mspace{14mu} {polymer}} \right)} = \frac{\left\lfloor {\left( {V_{eq} - V_{b}} \right) \cdot T} \right\rfloor \cdot 1000}{P}$

in which: V_(eq)=ml of KOH solution in ethanol at the end point for thetitration of the sample;

-   -   V_(b)=ml of KOH solution in ethanol required to reach a pH=9.5        during the blank titration;    -   T=concentration of the KOH solution in ethanol expressed as        mols/litre;    -   P=weight of the sample in grams.

When used for applications typical of plastics materials (such as forexample bubble film forming, injection moulding, expanded products,etc.), the Melt Flow Rate (MFR) for the polyesters obtained by theprocess according to this invention preferably lies between 500 and 1g/10 min, more preferably between 100 and 2 g/10 min, even morepreferably between 70 and 3 g/10 min (measurement made at 190° C./2.16kg according to ASTM standard D1238-89 “Standard Test Method for MeltFlow Rates of Thermoplastics by Extrusion Plastometer”). Preferably thepolyesters obtained by the process according to this invention have anintrinsic viscosity (measured using an Ubbelohde viscosity meter forsolutions in CHCl₃ having a concentration of 0.2 g/dl at 25° C.) of morethan 0.4, preferably between 0.4 and 2, more preferably between 0.7 and1.5 dl/g.

Preferably the polyesters obtained by the process according to thisinvention are biodegradable. In the meaning of this invention, bybiodegradable polymers are meant biodegradable polymers having arelative biodegradability after 180 days of 90% or more with respect tomicrocrystalline cellulose in accordance with standard ISO 14855-1(2013).

The polyesters obtained by the process according to this invention maybe used in a mixture, which may also be obtained by reactive extrusionprocesses, with one or more polymers of synthetic or natural origin,which may or may not be biodegradable.

In particular the polyesters obtained by the process according to thisinvention may be used as a mixture with biodegradable polyesters of thediacid-diol, hydroxyacid or polyester-ether type.

As far as the said biodegradable polyesters of the diacid-diol type areconcerned, these may be either aliphatic or aliphatic-aromatic.

The biodegradable aliphatic diacid-diol polyesters comprise aliphaticdiacids and aliphatic diols, while the biodegradable aliphatic-aromaticpolyesters have an aromatic part mainly comprising aromatic acids havingmultiple functional groups, the aliphatic part comprising aliphaticdiacids and aliphatic diols.

The biodegradable aliphatic-aromatic diacid-diol polyesters arepreferably characterised by a content of aromatic acids having multiplefunctional groups of between 30 and 90% mol, preferably between 45 and70% mol with respect to the total moles of the acid component.Preferably the aromatic acids having multiple functional groups areselected from aromatic dicarboxylic compounds of the phthalic acid typeand their esters, preferably terephthalic acid, and aromaticheterocyclic dicarboxylic acids and their esters, preferably2,5-furandicarboxylic acid. In a particularly preferred embodiment thesearomatic heterocyclic dicarboxylic compounds are obtained from rawmaterials of renewable origin, thus helping to reduce the utilisation ofnon-renewable resources such as, for example raw materials of fossilorigin.

Aliphatic-aromatic polyesters in which the aromatic acids havingmultiple functional groups comprise mixtures of aromatic dicarboxyliccompounds of the phthalic acid type and aromatic heterocyclicdicarboxylic compounds in which the aromatic heterocyclic dicarboxyliccompounds preferably comprise 1-99%, preferably 5-95%, more preferably20-90% mol with respect to the total moles of aromatic acids havingmultiple functional groups are particularly preferred.

The aliphatic diacids of biodegradable aliphatic and aliphatic-aromaticpolyesters comprise saturated dicarboxylic acids, such as oxalic acid,malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid,suberic acid, azelaic acid, sebacic acid, undecanoic acid, dodecanoicacid and brassylic acid, their esters and their mixtures. Of these,adipic acid and dicarboxylic acids from a renewal source are preferred,and among the latter dicarboxylic acids from renewable sources such assuccinic acid, sebacic acid, azelaic acid, undecanedioic acid,dodecanedioic acid and brassylic acid and their mixtures areparticularly preferred.

Examples of aliphatic diols in the biodegradable aliphatic andaliphatic-aromatic polyesters from diacid-diols are: 1,2-ethanediol,1,2-propanediol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol,1,6-hexanediol, 1,7-heptanediol, 1,8-octanediol, 1,9-nonanediol,1,10-decanediol, 1,11 -undecanediol, 1,12-dodecanediol, 1,13-tridecanediol, 1,4-cyclohexanedimethanol, neopentyl glycol,2-methyl-1,3-propanediol, dianhydrosorbitol, dianhydromannitol,dianhydroiditol, cyclohexanediol, cyclohexanemethanediol and theirmixtures. Of these, 1,4-butanediol, 1,3-propanediol and 1,2 ethanedioland their mixtures are particularly preferred.

Preferably the mixtures of polyesters obtained by the process accordingto this invention with the biodegradable aliphatic andaliphatic-aromatic polyesters from diacid-diols described above arecharacterised by a biodegradable polyester content which varies withinthe range between 1 and 99% by weight, more preferably between 5 and 95%by weight with respect to the sum of the weights of the polyestersobtained by the process according to this invention and the latterrespectively.

It is also possible to mix the polyesters according to the inventionwith more than one biodegradable polyester of the diacid-diol type. Bothbinary and ternary mixtures of polyesters obtained by the processaccording to this invention with the said biodegradable diacid-diolpolyesters are particularly preferred.

Of these biodegradable hydroxyacid polyesters, those preferred are:poly-L-lactic acid, poly-D-lactic and stereocomplex poly-D-L-lacticacid, poly-ε-caprolactone, polyhydroxybutyrate,polyhydroxybutyrate-valerate, polyhydroxybutyrate-propanoate,polyhydroxybutyrate-hexanoate, polyhydroxybutyrate-decanoate,polyhydroxybutyrate-dodecanoate, polyhydroxybutyrate-hexadecanoate,polyhydroxybutyrate-octadecanoate,poly-3-hydroxybutyrate-4-hydroxybutyrate.

Preferably the mixtures of polyesters obtained by the process accordingto this invention, with the biodegradable hydroxyacid polyestersdescribed above are characterised by a content of the said biodegradablepolyesters which varies within the range from 1 to 99% by weight,preferably from 5 to 95% by weight with respect to the sum of theweights of the polyesters obtained by the process according to thisinvention and the latter respectively.

The polyesters obtained by the process according to this invention mayalso be used as a mixture with polymers of natural origin such as, forexample starch, cellulose, chitin, chitosan, alginates, proteins such asgluten, zein, casein, collagen, gelatin, natural gums, rosinic acid andits derivatives, lignins as such, or purified, hydrolysed, basified,etc., lignins or their derivatives. Starches and cellulose may bemodified, and among these mention may be made of for example starch orcellulose esters having a degree of substitution of between 0.2 and 2.5,hydroxypropylate starches, starches modified with fatty chains, andcellophane. Mixtures with starch are particularly preferred. The starchmay be used in either destructured or gelatinised form or as a filler.The starch may represent the continuous or the disperse phase, or may bein a co-continuous form. In the case of dispersed starch, the starch ispreferably in a form smaller than a micron in average diameter, and morepreferably smaller than 0.5 μm.

Preferably the polyester mixtures obtained by the process according tothis invention together with the polymers of natural origin describedabove are characterised by a content of the said polymers of naturalorigin which varies within the range from 1 to 99% by weight, morepreferably between 5 and 95% by weight, and more preferably 10 and 40%by weight with respect to the sum of the weights of the polyestersobtained by the process according to this invention and the latterrespectively.

The polyesters obtained by the process according to this invention mayalso be used as a mixture with polyolefins, aromatic polyesters,polyester- and polyether-urethanes, polyurethanes, polyamides,polyaminoacids, polyethers, polyureas, polycarbonates and mixturesthereof.

Among the polyolefins, those preferred are: polyethylene, polypropylene,their copolymers, polyvinyl alcohol, polyvinyl acetate, poly ethyl vinylacetate and polyethylene vinyl alcohol. Of the aromatic polyesters,those preferred are: PET, PBT, PTT in particular having a renewablecontent of >30% and polyalkylenefurandicarboxylates. Of the latter,those preferred are: poly(1,2-ethylene-2,5 -furandicarboxylate),poly(1,3 -propylene-2,5 -furandicarboxylate),poly(1,4-butylene-2,5-furandicarboxylate) and their mixtures. Examplesof polyamides are: polyamide 6 and 6.6, polyamide 9 and 9.9, polyamide10 and 10.10, polyamide 11 and 11.11, polyamide 12 and 12.12 and theircombinations of the 6/9, 6/10, 6/11, 6/12 type.

The polycarbonates may be polyethylene carbonates, polypropylenecarbonates, polybutylene carbonates, their mixtures and copolymers.

The polyethers may be polyethylene glycols, polypropylene glycols,polybutylene glycols, their copolymers and their mixtures havingmolecular weights from 70000 to 500000.

Preferably the polyester mixtures obtained by the process according tothis invention using the polymers described above (polyolefins, aromaticpolyesters, polyester- and polyether-urethanes, polyurethanes,polyamides, polyaminoacids, polyethers, polyureas, polycarbonates andmixtures thereof) are characterised by a content of the said polymerswhich varies within the range between 0.5 and 99% by weight, morepreferably between 5 and 50% by weight with respect to the sum of theweights of the polyesters obtained by the process according to thisinvention and the latter respectively.

The polyesters obtained by the process according to this invention areextremely suitable for use alone or as a mixture in other polymers, inmany practical applications for the manufacture of products such as, forexample films, fibres, non-woven fabrics, sheets, moulded, thermoformed,blown, expanded and laminated articles, including those manufactured bythe technique of extrusion coating.

Examples of products comprising the polyesters obtained by the processaccording to this invention are:

-   -   films, with mono and bi-orientated films, and multi-layer films        with other polymer materials;    -   film for use in the agricultural sector as mulching films;    -   stretch films including thin film for foodstuffs, for bales in        agriculture and for the wrapping of wastes;    -   bags and linings for the collection of organic materials such as        the collection of food wastes and grass cuttings;    -   thermoformed food packages, both single-layer and multi-layer,        such as for example containers for milk, yogurt, meat,        beverages, etc.;    -   coatings obtained by the technique of extrusion coating;    -   multi-layer laminates with layers of cardboard, plastics        materials, aluminium, metallised films;    -   expanded or expandable beads for the production of parts formed        by sintering;    -   expanded and semi-expanded products including expanded blocks        formed by pre-expanded particles;    -   expanded sheets, thermoformed expanded sheets, containers from        these obtained for food packaging;    -   containers in general for fruit and vegetables;    -   compositions with gelatinised, destructured and/or complex        starch, natural starch, flours, other fillers of natural, plant        or inorganic origin as filler;    -   fibres, microfibres, composite fibres with a core comprising        rigid polymers such as PLA, PET, PTT, etc., and an outer shell        of the material according to the invention, dablens composite        fibres, fibres having various cross-sections from round to        multi-lobed, flock fibres, fabrics and non-woven or spun-bonded        or thermally bonded fabrics for the sanitary, hygiene,        agriculture and clothing sectors.

They may also be used in applications as a replacement for plasticisedPVC.

The process according to this invention comprises an esterification ortransesterification stage and a polycondensation stage, and ischaracterised in that the said polycondensation stage is carried out inthe presence of a catalyst comprising a mixture of at least oneTitanium-based compound and at least one Zirconium-based compound inwhich the Ti/(Ti+Zr) weight ratio is equal to or greater than 0.01 andequal to or less than 0.7, preferably equal to or greater than 0.02 andequal to or less than 0.60.

The esterification/transesterification stage is preferably fed with amolar ratio between the aliphatic diols and the dicarboxylic acids,their esters and their salts, which is preferably between 1 and 2.5,preferably between 1.05 and 1.7.

The dicarboxylic acids, their esters or their salts, the aliphatic diolsand any other co-monomers which constitute the polyester may be fed tothe said stage separately, thus becoming mixed in the reactor, or mayalternatively be premixed, preferably at T <70° C., before beingdelivered to the reactor. It is also possible to premix part of thecomponents and subsequently modify their composition, for example in thecourse of the esterification/transesterification reaction.

In the case of polyesters in which the dicarboxylic component comprisesrepeating units deriving from several dicarboxylic acids, whether theseare aliphatic or aromatic, it is also possible to premix some of thesewith aliphatic diols, preferably at T <70° C., adding the remainingportion of the dicarboxylic acids, diols and any other co-monomers tothe esterification/transesterification reactor.

The esterification/transesterification stage of the process according tothis invention is advantageously carried out at a temperature of200-250° C. and a pressure of 0.7-1.5 bar, preferably in the presence ofa esterification/transesterification catalyst.

The esterification/transesterification catalyst, which may alsoadvantageously be used as a component of the polycondensation stagecatalyst, may in turn be fed directly to theesterification/transesterification reactor or may be first alsodissolved in an aliquot of one or more of the dicarboxylic acids, theiresters or their salts, and the aliphatic diols, in such a way as to aiddispersion in the reaction mixture and render it more uniform. In apreferred embodiment the esterification/transesterification catalyst isselected from organometallic compounds of Tin, for example, stannoicacid derivatives, Titanium compounds, for example, titanates such astetrabutyl orthotitanate or tetra(isopropyl) orthotitanate, Zirconiumcompounds, for example zirconates such as tetrabutyl orthozirconate ortetra(isopropyl) orthozirconate, compounds of Antimony, Cobalt, Lead,Aluminium, for example Al-triisopropyl and Zinc compounds and mixturesthereof.

With regard to the organometallic esterification/transesterificationcatalysts of the type mentioned above, during theesterification/transesterification stage of the process according tothis invention they are present in concentrations preferably between 12and 120 ppm of metal with respect to the quantity of polyester which cantheoretically be obtained by converting all of the dicarboxylic acid fedto the reactor.

In a preferred embodiment the catalyst for theesterification/transesterification stage is a titanate, more preferablydiisopropyl, triethanolamino titanate, preferably used in aconcentration of 12-120 ppm of metal with respect to a quantity ofpolyester which can theoretically be obtained by converting all of thedicarboxylic acid fed to the reactor. Preferably the reaction time forthe esterification/transesterification stage in the process according tothis invention is between 4 and 8 hours. At the end of theesterification/transesterification stage an oligomer product having Mn<5000, an intrinsic viscosity of 0.05-0.15 dl/g, and an acidity <80meq/kg is obtained.

In a preferred embodiment of the process according to this invention thecatalyst is fed to the polycondensation stage together with the oligomerproduct at the end of the esterification/transesterification stage.

The polycondensation stage in the process according to this invention iscarried out in the presence of a catalyst comprising a mixture of atleast one Titanium-based compound and at least one Zirconium-basedcompound in which the Ti/(Ti+Zr) ratio by weight is equal to or greaterthan 0.01 and equal to or less than 0.70, preferably equal to or greaterthan 0.02 and equal to or less than 0.60.

In a preferred embodiment the polycondensation catalyst based onTitanium is a titanate advantageously selected from compounds having thegeneral formula Ti(OR)₄ in which R is a ligand group comprising one ormore atoms of Carbon, Oxygen, Phosphorus, Silicon and/or Hydrogen.Different ligand groups R may be present on the same Titanium atom, butpreferably they are identical so as to assist preparation of thetitanate. Also, 2 or more ligands R may be derived from a singlecompound and may be chemically bound together in addition to being boundby the Titanium (so-called multidentate ligands such as for exampletriethanolamine, citric acid, glycolic acid, malic acid, succinic acid,ethanediamine). R is advantageously selected from H, triethanolamine,citric acid, glycolic acid, malic acid, succinic acid, 3-oxobutanoicacid, ethanediamine and branched C₁-C₁₂ alkyl residues such as forexample ethyl, propyl, n-butyl, pentyl, isopropyl, isobutyl, isopentyl,hexyl and ethylhexyl. In a preferred embodiment R is selected fromC₁-C₁₂ alkyl residues, preferably C₁-C₈, more preferably n-butyl.

The preparation of titanates is known in the literature. Typically theseare prepared by causing Titanium tetrachloride and the precursor alcoholof formula ROH to react in the presence of a base such as for exampleammonia, or through the transesterification of other titanates.Commercial examples of titanates which it is possible to use in theprocess according to this invention include the products Tyzor ® TPT(tetra isopropyl Titanate), Tyzor ® TnBT (tetra n-butyl Titanate) andTyzor ® TE (diisopropyl triethanolamino Titanate).

In a preferred embodiment the polycondensation catalyst based onZirconium is a zirconate advantageously selected from compounds havingthe general formula Zr(OR)₄ in which R is a ligand group comprising oneor more atoms of Carbon, Oxygen, Phosphorus, Silicon and/or Hydrogen. Asin the case of titanates, several different ligand groups R may bepresent on the same Zirconium atom, but preferably these are identicalso as to assist preparation of the zirconate. In addition to this, 2 ormore ligands R may be derived from a single compound or may bechemically bound together in addition to being bound by the Zirconium(so-called multidentate ligands such as for example triethanolamine,citric acid, glycolic acid, malic acid, succinic acid, ethanediamine). Ris advantageously selected from H, triethanolamine, citric acid,glycolic acid, malic acid, succinic acid, 3-oxobutanoic acid,ethanediamine and straight or branched C₁-C₁₂ alkyl residues such as forexample ethyl, propyl, n-butyl, pentyl, isopropyl, isobutyl, isopentyl,hexyl or ethylhexyl. In a preferred embodiment, R is selected fromC₁-C₁₂ alkyl residues, preferably C₁-C₈, more preferably n-butyl.

The preparation of zirconates is known in the literature, and is similarto that described above for titanates. Commercial examples of zirconateswhich can be used in the process according to this invention include theproducts Tyzor ® NBZ (tetra n-butyl Zirconate), Tyzor NPZ (tetran-propyl Zirconate), IG-NBZ (tetra n-butyl Zirconate)

Preferably the catalyst in the polycondensation stage of the processaccording to this invention comprises a mixture of at least one titanateand at least one zirconate, more preferably a mixture of tetra n-butylTitanate and tetra n-butyl Zirconate.

In addition to Titanium and Zirconium compounds, the polycondensationcatalyst may also comprise phosphorus compounds, for example phosphonicand phosphinic acids, organic phosphates and phosphites, Silicates,organic and inorganic salts of alkali metals and alkaline earth metals.

The polycondensation catalyst may be fed to the polycondensation stageeither by separately feeding its various components to the reactor, orpremixing them and feeding them to the reactor as a mixture. It is alsopossible to premix some of the components and adjust the catalystcomposition subsequently, for example at the time when it is placed incontact with the oligomer product.

When a catalyst containing Titanium and/or Zirconium compounds is usedin the esterification/transesterification stage of the process accordingto this invention, in a preferred embodiment of the process according tothis invention this catalyst is not separated from the oligomer productand is fed together with it to the polycondensation stage andadvantageously used as a polycondensation catalyst or as a componentthereof, with possible adjustment of the molar ratio between Titaniumand Zirconium by adding suitable quantities of Titanium and Zirconiumcompounds to the said polycondensation stage. In a particularlypreferred embodiment the catalyst for the polycondensation stage is thesame as that for the esterification/transesterification stage.

The polycondensation stage is advantageously carried out by feeding theoligomer product to the polycondensation reactor and causing the wholeto react in the presence of the catalyst at a temperature of 220-250° C.and at a pressure of <5mbar.

Preferably the polycondensation stage of the process according to thisinvention is carried out in the presence of a total quantity of Titaniumand Zirconium of 80-500 ppm, with respect to the quantity of polyesterwhich could theoretically be obtained by converting all of thedicarboxylic acid fed to the reactor in the catalyst.

Preferably the reaction time for the polycondensation stage in theprocess according to this invention is between 4 and 8 hours. At the endof the polycondensation stage a polyester having Mn >50000, an intrinsicviscosity of 0.9-1.05 dl/g, and an acidity <50 meq/kg is obtained.

Dependent upon the specific molecular weight properties and the desiredviscosity for the polyester, the process according to this invention mayprovide for one or more stages of chain extension, reactive processingor reactive extrusion, including with other polymers through the use ofperoxides, divinyl ethers, bisoxazoline, polyepoxides, di- andpoly-isocyanates, carbodiimides or dianhydrides after thepolycondensation stage.

The invention will now be illustrated through a number of embodimentswhich are intended to be by way of example and not limiting the scope ofprotection of this patent application.

EXAMPLES Example 1 (Comparative) Preparation of a poly (1,4-butylenesebacate-co-1,4-butylene terephthalate) (PBST) with 48% mol of1,4-butylene terephthalate Units Using a Polycondensation CatalystComprising Only Titanate

Esterification Stage

6677 g of terephthalic acid, 8802 g of sebacic acid, 11313 g of1,4-butanediol, 11.6 g of glycerine and 3.4 g of an 80% by weightethanolic solution of diisopropyl triethanolamino Titanate (Tyzor TE,containing 8.2% by weight of Titanium) were added in a diol/dicarboxylicacid molar ratio (MGR) of 1.50 to a steel reactor having a geometricalcapacity of 24 litres, fitted with a mechanical stirrer system, an inletfor nitrogen, a distillation column, a knock-down system for high-volumedistillates and a connection to a high vacuum system.

The temperature of the mass was gradually increased to 230° C. over aperiod of 120 minutes.

Polycondensation Stage

When 95% of the theoretical water had been distilled off, 17.2 g(corresponding to 120 ppm of metal with respect to the quantity of PBSTwhich could theoretically be obtained by converting all the sebacic acidand all the terephthalic acid fed to the reactor) of tetra n-butylTitanate was added, thus obtaining a Ti/(Zr+Ti) ratio by weight of 1,bearing in mind the Titanium introduced with the esterificationcatalyst. The temperature of the reactor was then raised to 235-240° C.and the pressure was gradually reduced until a value of less than 2 mbarwas reached over a period of 60 minutes. The reaction was allowed toproceed for approximately 4:30 hours, the time required to obtain a PBSTwith an MFR of approximately 3-4 (g/10 minutes at 190° C. and 2.18 kg),and the material was then discharged into a water bath in the form of astring and granulated.

Example 2 (Comparative) Preparation of a PBST with 48% mol of1,4-butylene terephthalate Units Using a Polycondensation CatalystComprising a Titanate and Zirconate Mixture with a Ti/(Zr+Ti) Ratio byWeight=0.75

Example 1 was repeated adding 18.12 g (corresponding to 139 ppm of metalwith respect to the quantity of PBST which could theoretically beobtained by converting all the sebacic acid and all the terephthalicacid fed to the reactor) of a mixture of 14.42 g of tetra n-butylTitanate and 3.7 g of tetra n-butyl Zirconate to the polycondensationstage instead of 17.2 g of tetra n-butyl Titanate, thus obtaining aTi/(Zr+Ti) ratio by weight of 0.75, bearing in mind the Titaniumintroduced with the esterification catalyst.

Example 3 Preparation of a PBST with 48% mol of 1,4-butyleneterephthalate Units Using a Polycondensation Catalyst Comprising aTitanate and Zirconate Mixture with a Ti/(Zr+Ti) ratio by Weight =0.60

Example 1 was repeated adding 18.74 g (corresponding to 153 ppm of metalwith respect to the quantity of PBST which could theoretically beobtained by converting all the sebacic acid and all the terephthalicacid fed to the reactor) of a mixture of 12.28 g of tetra n-butylTitanate and 6.46 g of tetra n-butyl Zirconate to the polycondensationstage instead of 17.2 g of tetra n-butyl Titanate, thus obtaining aTi/(Zr+Ti) ratio by weight of 0.604, bearing in mind the Titaniumintroduced with the esterification catalyst.

Example 4 Preparation of a PBST with 48% mol of 1,4-butyleneterephthalate Units Using a Polycondensation Catalyst Comprising aTitanate and Zirconate mixture with a Ti/(Zr+Ti) Ratio by Weight =0.24

Example 1 was repeated adding 20.8 g (corresponding to 198 ppm of metalwith respect to the quantity of PBST which could theoretically beobtained by converting all the sebacic acid and all the terephthalicacid fed to the reactor) of a mixture of 5.2 g of tetra n-butyl Titanateand 15.6 g of tetra n-butyl Zirconate to the polycondensation stageinstead of 17.2 g of tetra n-butyl Titanate, thus obtaining a Ti/(Zr+Ti)ratio by weight of 0.24, bearing in mind the Titanium introduced withthe esterification catalyst.

Example 5 Preparation of a PBST with 48% mol of 1,4-butyleneterephthalate Units Using a Polycondensation Catalyst Comprising aTitanate and Zirconate Mixture with a Ti/(Zr+Ti) Ratio by Weight =0.10

Example 1 was repeated adding 21.92 g (corresponding to 222 ppm of metalwith respect to the quantity of PBST which could theoretically beobtained by converting all the sebacic acid and all the terephthalicacid fed to the reactor) of a mixture of 1.34 g of tetra n-butylTitanate and 20.58 g of tetra n-butyl Zirconate to the polycondensationstage instead of 17.2 g of tetra n-butyl Titanate, thus obtaining aTi/(Zr+Ti) ratio by weight of 0.10, bearing in mind the Titaniumintroduced with the esterification catalyst.

Example 6 (Comparative) Preparation of a PBST with 48% mol of1,4-butylene terephthalate Units Using a Polycondensation CatalystComprising only a Zirconate

Example 1 was repeated by adding in theesterification/transesterification stage, instead of 3.4 g of an 80% byweight ethanolic solution of diisopropyl triethanolamino Titanate , 5.94g of Ammonium Zirconium Lactate and adding 22.10g (corresponding to 229ppm of metal with respect to the quantity of poly-1,4-butylene succinatewhich could theoretically be obtained by converting all of the succinicacid fed to the reactor) of g of tetra n-butyl Zirconate in thepolycondensation stage, instead of 17.2 g of tetra n-butyl Titanate,thus obtaining a Ti/(Zr+Ti) ratio by weight of 0.

Example 7 (Comparative) Preparation of a PBST with 85% mol of1,4-butylene terephthalate Units Using a Polycondensation CatalystComprising only a Titanate and Zirconate mixture with a Ti/(Zr+Ti) ratioby weight =0.75

Example 2 was repeated for preparing a PBST with 85% mol of 1,4-butyleneterephthalate units, instead of a PBTS with 48 mol % of 1,4-butyleneterephthalate units. Thus, 12520 g of terephthalic acid, 2690 g ofsebacic acid, 11980 g of 1,4-butanediol, 12.2 g of glycerine were addedto the esterification/transesterification stage,

Example 8 (Comparative) Preparation of a PBST with 85 mol % of AromaticUnits using a Polycondensation Catalyst Comprising only a Titanate andZirconate Mixture with a Ti/(Zr+Ti) Ratio by Weight =0.24.

Example 4 was repeated for preparing a PBST with 85% mol of 1,4-butyleneterephthalate units, instead of a PBTS with 48 mol % of 1,4-butyleneterephthalate units. Thus, 12520 g of terephthalic acid, 2690 g ofsebacic acid, 11980 g of 1,4-butanediol, 12.2 g of glycerine were addedto the esterification/transesterification stage,

Example 9 Preparation of a poly (1,4-butylene adipate-co-1,4-butyleneterephthalate) (PBAdT) with 47% mol of 1,4-butylene terephthalate UnitsUsing a Polycondensation Catalyst Comprising a Titanate and ZirconateMixture with a Ti/(Zr+Ti) ratio by weight =0.60

Example 3 was repeated for preparing a PBAdT with 47% mol of1,4-butylene terephthalate units, instead of a PBTS with 48 mol % of1,4-butylene terephthalate units. Thus, 7450 g of terephthalic acid,7390 g of adipic acid, 12890 g of 1,4-butanediol, 13.2 g of glycerinewere added to the esterification/transesterification stage.

Example 10 Preparation of a PBAdT with 47% mol of 1,4-butyleneterephthalate Units Using a Polycondensation Catalyst Comprising aTitanate and Zirconate Mixture with a Ti/(Zr+Ti) Ratio by Weight =0.24

Example 4 was repeated for preparing a PBAdT with 47% mol of1,4-butylene terephthalate units, instead of a PBTS with 48 mol % of1,4-butylene terephthalate units. Thus, 7450. g of terephthalic acid,7390 g of adipic acid, 12890 g of 1,4-butanediol, 13.2 g of glycerinewere added to the esterification/transesterification stage.

Example 11 Preparation of a PBAdT with 47% mol of 1,4-butyleneterephthalate Units Using a Polycondensation Catalyst Comprising aTitanate and Zirconate Mixture with a Ti/(Zr+Ti) ratio by Weight =0.10

Example 5 was repeated for preparing a PBAdT with 47% mol of1,4-butylene terephthalate units, instead of a PBTS with 48 mol % of1,4-butylene terephthalate units. Thus, 7450 g of terephthalic acid,7390 g of adipic acid, 12890 g of 1,4-butanediol, 13.2 g of glycerinewere added to the esterification/transesterification stage.

Example 12 (Comparative) Preparation of a PBAdT with 47% mol of1,4-butylene terephthalate Units Using a Polycondensation CatalystComprising only a Zirconate

Example 6 was repeated for preparing a PBAdT with 47% mol of1,4-butylene terephthalate units, instead of a PBTS with 48 mol % of1,4-butylene terephthalate units. Thus, 7450 g of terephthalic acid,7390 g of adipic acid, 12890 g of 1,4-butanediol, 13.2 g of glycerinewere added to the esterification/transesterification stage.

Example 13 Preparation of a poly (1,4-butylene azelate-co-1,4-butyleneterephthalate) (PBAzT) with 48% mol of 1,4-butylene terephthalate UnitsUsing a Polycondensation Catalyst Comprising a Titanate and ZirconateMixture with a Ti/(Zr+Ti) ratio by Weight =0.60

Example 3 was repeated for preparing a PBAzT with 48% mol of1,4-butylene terephthalate units, instead of a PBTS with 48 mol % of1,4-butylene terephthalate units. Thus, 6910 g of terephthalic acid,8480 g of adipic acid, 11700 g of 1,4-butanediol, 12.0 g of glycerinewere added to the esterification/transesterification stage.

Example 14 Preparation of a PBAzT with 48% mol of 1,4-butyleneterephthalate Units Using a Polycondensation Catalyst Comprising aTitanate and Zirconate Mixture with a Ti/(Zr+Ti) Ratio by Weight =0.24

Example 4 was repeated for preparing a PBAzT with 48% mol of1,4-butylene terephthalate units, instead of a PBTS with 48 mol % of1,4-butylene terephthalate units. Thus, 6910 g of terephthalic acid,8480 g of adipic acid, 11700 g of 1,4-butanediol, 12.0 g of glycerinewere added to the esterification/transesterification stage.

Example 15 Preparation of a PBAzT with 48% mol of 1,4-butyleneterephthalate Units Using a Polycondensation Catalyst Comprising aTitanate and Zirconate Mixture with a Ti/(Zr+Ti) Ratio by Weight =0.10

Example 5 was repeated for preparing a PBAzT with 48% mol of1,4-butylene terephthalate units, instead of a PBTS with 48 mol % of1,4-butylene terephthalate units. Thus, 6910. g of terephthalic acid,8480 g of adipic acid, 11700 g of 1,4-butanediol, 12.0 g of glycerinewere added to the esterification/transesterification stage.

Example 16 (Comparative) Preparation of a PBAzT with 48% mol of1,4-butylene terephthalate Units Using a Polycondensation CatalystComprising only a Zirconate

Example 6 was repeated for preparing a PBAzT with 48% mol of1,4-butylene terephthalate units, instead of a PBTS with 48 mol % of1,4-butylene terephthalate units. Thus, 6910. g of terephthalic acid,8480 g of adipic acid, 11700 g of 1,4-butanediol, 12.0 g of glycerinewere added to the esterification/transesterification stage.

Samples of the polyesters according to examples 1-16 were obtained atthe end of the stage of discharging the reactor to determine their MFR,oligomer content and terminal acid groups content (CEG) in accordancewith the methods described in this application (Table 1).

TABLE 1 Examples 1-16 Moles MFR mol % of Ti/ of (g/10 min 1,4-butylene(Ti + Zr) Metal/ Reaction 190° C. CEG terephthalate (weight/ ton of timeOligomers and (meq/ Example Polyester units MGR weight) polyester(hrs:mins) (%) 2.16 kg) kg)  1 (comparative) PBST 48 1.50 1 2.81 4:305.0 3.3 36  2 (comparative) PBST 48 1.50 0.75 2.83 4:30 5.0 5.2 38  3PBST 48 1.50 0.60 2.83 3:20 2.2 3.7 34  4 PBST 48 1.50 0.24 2.82 5:003.5 4.4 32  5 PBST 48 1.50 0.10 2.83 6:30 2.1 3.5 28  6 (comparative)PBST 48 1.50 0 2.83 7:45 1.4 >40 nd  7 (comparative) PBST 85 1.50 0.752.83 3:00 0.7 16.6 27 (220° C./ 2.16 Kg)  8 (comparative) PBST 85 1.500.24 2.83 2:45 0.5 17.4 24 (220° C./ 2.16 Kg)  9 PBAdT 47 1.50 0.60 2.476:00 1.7 10 16 10 PBAdT 47 1.50 0.24 2.47 4:00 1.2 11 17 11 PBAdT 471.50 0.10 2.47 5:50 1.0 11.9 15 12 (comparative) PBAdT 47 1.50 0 2.478:00 1.0 >40 nd 13 PBAzT 48 1.50 0.60 2.83 4:00 1.9 3.6 43 14 PBAzT 481.50 0.24 2.83 4:00 2.6 5.6 32 15 PBAzT 48 1.50 0.10 2.83 7:50 1.3 7.535 16 (comparative) PBAzT 48 1.50 0 2.83 10:00  1.0 >40 nd

1. A process for the production of polyesters comprising: a) adicarboxylic component comprising: a1) 40-60% mol of units deriving fromat least one aromatic dicarboxylic acid, a2) 60-40% mol of unitsderiving from at least one aliphatic dicarboxylic acid, b) a diolcomponent comprising units deriving from at least one aliphatic diol;said process comprising an esterification step and a followingpolycondensation step, wherein said polycondensation step is performedin the presence of a catalyst comprising a mixture of at least oneTitanium-based compound and at least one Zirconium-based compound inwhich the weight ratio Ti/(Ti+Zr) is equal to or higher than 0.01 andequal to or lower than 0.70.
 2. The process according to claim 1, inwhich said aliphatic diol is selected from the group consisting of1,2-ethanediol, 1,2-propanediol, 1,3-propanediol, 1,4-butanediol,1,5-pentanediol, 1,6-hexanediol, 1,7-hheptanediol, 1,8-octanediol,1,9-nonanediol, 1,10-decanediol, 1,11-undecanediol, 1,12-dodecanediol,1,13-tridecanediol, 1,4-cyclohexanedimethanol, neopentylglycol,2-methyl-1,3-propanediol, dianhydrosorbitol, dianhydromannitol,dianhydroiditol, cyclohexanediol, cyclohexanemethanediol,pentaerythritol, glycerol, polyglycerol, trimethylolpropane,polyalkylene glycols with molecular weight of 100-4000 and mixturesthereof.
 3. The process claim 1, in which said aromatic dicarboxylicacid is selected from the group consisting of terephthalic acid,isophthalic acid, 2,5-furandicarboxylic acid, their esters, their saltsand mixtures thereof.
 4. The process claim 1, in which said aliphaticdicarboxylic acid is selected from the group consisting of saturateddicarboxylic acids C₂-C₂₄, their alkylic esters C₁-C₂₄, their salts andmixtures thereof.
 5. The process claim 1, in which said esterificationstep the molar ratio between the, aliphatic diols and the dicarboxylicacids, their ester and their salts is between 1 and 2.5.
 6. The processclaim 1, in which the esterification step is performed at a temperatureof 200-250° C. and at a pressure of 0.7-1.5 bar.
 7. The process claim 1,in which the esterification step is performed in the presence of anorganometallic catalyst selected from the group consisting of theorganometallic compounds of Tin, Titanium, Zirconium, Antimony, Cobalt,Lead, Aluminium, Zinc and mixtures thereof.
 8. The process according toclaim 7, in which said organometallic catalyst is present, in theesterification step, in a concentration of 12-120 ppm of metal withrespect to the amount of polyester theoretically obtainable byconversion of all the dicarboxylic acid fed to the process.
 9. Theprocess claim 1, in which said Titanium-based compound of thepolycondensation step is a Titanate having general formula Ti(OR)₄ inwhich R is a ligand group comprising one or more atoms of Carbon, OxygenPhosphorus, Silica, and/or Hydrogen.
 10. The process according to claim9, in which said R is selected from the group consisting of H,triethanolamine, citric acid, glycolic acid, malic acid, succinic acid,ethanediamine, linear and branched alkyl residues C₁-C₁₂.
 11. Theprocess according to claim 10, in which said R is selected from linearor branched alkyl residues C₁-C₁₂.
 12. The process according to claim11, in which said R is n-butyl.
 13. The process claim 1, in which saidZirconium-based compound of the polycondensation phase is a Zirconatehaving general formula Zr(OR)₄ in which R is a ligand group comprisingone or more atoms of Carbon, Oxygen Phosphorus, Silica, and/or Hydrogen.14. The process according to claim 13, in which said R is selected fromthe group consisting of H, triethanolamine, citric acid, glycolic acid,malic acid, succinic acid, ethanediamine, linear and branched alkylresidues C₁-C₁₂.
 15. The process according to claim 14, in which said Ris selected from linear or branched alkyl residues C₁-C₁₂.
 16. Theprocess according to claim 15, in which said R is n-butyl.
 17. Theprocess claim 1, in which said polycondensation catalyst comprises amixture of tetra n-butyl Titanate and tetra n-butyl Zirconate.
 18. Theprocess claim 1, in which said polycondensation step is performed at atemperature of 220-250 ° C. and at a pressure <5 mbar.
 19. The processclaim 1, in which said polycondensation step is performed in thepresence of a total amount of Titanium and Zirconium-based catalyst of80-500 ppm of metal, with respect to the amount of polyestertheoretically obtainable by conversion of all the dicarboxylic acid fedto the process.
 20. The process claim 1, comprising, after thepolycondensation step, one or more steps of chain extension, reactiveprocessing and/or of reactive extrusion.
 21. A use of a catalystcomprising a mixture of at least one Titanium-based compound and atleast one Zirconium-based compound in which the weight ratio Ti/(Ti+Zr)is equal to or higher than 0.01 and equal to or lower than 0.70 forproducing polyesters comprising: a) a dicarboxylic component comprising:a1) 40-80% mol of units deriving from at least one aromatic dicarboxylicacid, a2) 20-60% mol of units deriving from at least one aliphaticdicarboxylic acid, b) a diol component comprising units deriving from atleast one aliphatic diol.